The presently disclosed embodiments relate in general to electrostatography comprising improved features in the flexible imaging member that enhance function when used in the electrostatographic imaging system. These embodiments pertain, more particularly, to a structurally simplified curl-free flexible electrostatographic imaging member belt containing a stress/strain free ground strip layer to improve dynamic belt cyclic motion quality and extend service life. The present disclosure relates to all types of flexible electrophotographic imaging member belts used in electrophotography.
Electrostatographic imaging members are known in the art. Typical electrostatographic imaging members include (1) electrophotographic imaging members or photoreceptors for electrophotographic imaging systems and (2) electroreceptors such as ionographic imaging members for electrographic imaging systems. Generally, these imaging members comprise at least a supporting substrate and at least one imaging layer comprising a thermoplastic polymeric matrix material. In an electrophotographic imaging member or photoreceptor, the photoconductive imaging layer may comprise only a single photoconductive layer or multiple of layers such as a combination of a charge generating layer and one or more charge transport layer(s). In an electroreceptor, the imaging layer is a dielectric imaging layer.
Electrostatographic imaging members can have a number of distinctively different configurations. For example, they can comprise a flexible member, such as a flexible scroll or a belt containing a flexible substrate. Since typical flexible electrostatographic imaging members exhibit spontaneous upward imaging member curling after completion of solution coating the outermost exposed imaging layer, an anticurl back coating is required to be applied to back side of the flexible substrate support to counteract/balance the curl and provide the desirable imaging member flatness. Alternatively, the electrostatographic imaging members can also be a rigid member, such as those utilizing a rigid substrate support drum. For these drum imaging members, having a thick rigid cylindrical supporting substrate bearing the imaging layer(s), there is no exhibition of the curl-up problem, and thus, there is no need for an anticurl back coating layer. Consequently, these drum imaging members are not included in the scope of the present disclosure.
In the present disclosure, methodology and material compositions used for reformulations (pertaining to structurally simplified flexible electrostatographic imaging members that have virtually curl-free configuration without the need for an anticurl back coating) are detailed. The prepared imaging members having functionally improved the outermost exposed layers do provide an extended useful service-life function and the process for making and using these members as specified are equality applicable for flexible imaging members in all varieties of form. Even though the electrostagraphic imaging members of these disclosures relate to both electrophotographic imaging members (or photoreceptors) and ionographic imaging members (electroreceptors) in each respective flexible belt configuration, nonetheless, for reason of simplicity, all the disclosed embodiments detailed hereinafter are focused and represented primarily on the electrophotographic imaging members in flexible belt configuration which are for use in electrophotography.
A number of current flexible electrophotographic imaging member belts are multilayered photoreceptor belts that, in a negative charging system, comprise a substrate support, an electrically conductive layer, an optional charge blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer having a co-coated ground strip layer adjacent to the charge transport layer and at one edge of the belt, and an anticurl back coating at the opposite side of the substrate support. Since these flexible electrophotographic imaging member belts do always exhibit upward curling after completing the solution application coating process of a charge transport layer and the co-coated ground strip layer, the anticurl back coating is needed on the back side of the flexible substrate support (the side opposite from the electrically active layers) to balance/control the curl and render the imaging member belts flatness. So in the current imaging member belt design, the charge transport layer and the its adjacent ground strip layer at one edge of the photoreceptor belt are the two top outermost layers that are constantly exposed to the environment contaminants as well as the machine subsystems interactions during electrophotographic imaging and cleaning processes, while the anticurl back coating is the bottom outermost exposed layer subjected to belt support module components action under dynamic belt cycling condition.
The flexible electrophotographic imaging member belts are generally prepared in a seamed or seamless belt configuration. Flexible electrophotographic imaging member seamed belts are typically fabricated from a sheet which is cut from a web. The sheets are generally rectangular in shape. The edges may be of the same length or one pair of parallel edges may be longer than the other pair of parallel edges. The sheets are formed into a belt by joining overlapping opposite marginal end regions of the sheet. A seam is typically produced in the overlapping marginal end regions at the point of joining. Joining may be effected by any suitable means. Typical joining techniques include welding (including ultrasonic), gluing, taping, pressure heat fusing, and the like. Ultrasonic welding is generally the more desirable method of joining because it is rapid, clean (no solvents) and produces a thin and narrow seam. In addition, ultrasonic welding is more desirable because it causes generation of heat at the contiguous overlapping end marginal regions of the sheet to maximize melting of one or more layers therein to produce a strong fusion bonded seam.
In a typical negative charging machine design, the prepared flexible imaging member seamed belt is mounted over and encircled around a belt support module comprising numbers of belt support rollers and backer bars ready for electrophotographic imaging function. The flexible electrophotographic imaging member seamed belt is imaged by uniformly depositing an electrostatic charge on the imaging surface of the electrophotographic imaging member and then exposing the imaging member to a pattern of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the imaging member while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking toner particles on the imaging member surface. The resulting visible toner image can then be transferred to a suitable receiving member or substrate such as paper. Therefore, under these normal machine operation conditions in the field, the flexible imaging member seamed belt is in dynamic fatigued cyclic motion during electrophotographic image printing processes. The top outermost exposed charge transport layer and the adjacent ground strip layer of the imaging member belt are therefore constantly in intimate mechanical interactions with various machine subsystems and components (such as for example cleaning blade, tab blade, cleaning brush on the charge transport layer and belt edge guides on the ground strip layer, etc.) and chemical exposure (such as corona effluents from charging devices) to therefore causing fatigue and degradation of these layers. As a consequence, these interactions have been seen to facilitate and exacerbate the early development of two crucial charge transport layer material failures in the belt, causing copy printout defects to premature cut short its service life prior to reaching the intended belt life target. Moreover, under the machine imaging member belt functioning conditions, the bottom outermost exposed the anticurl back coating is constantly subjected to belt support rollers and backer bars mechanical interactions which thereby promoting on-set of premature anticurl back coating wear and abrasion streaking failures.
Typical negatively-charged electrophotographic imaging members, such as the flexible photoreceptor designs, are made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and a conductive ground strip layer co-coated adjacently to the charge transport layer at one edge of the imaging member. During the imaging member web extrusion co-coating process, the ground strip layer solution and the charge transport layer solution are simultaneously applied, adjacent to each other, over the charge generating layer with the ground strip layer at the edge of the imaging member web. The charge transport layer and the co-coated ground strip layers are usually the last layers, or the outermost layers, to be coated and are applied by solution co-coating coating process, then followed by drying the wet applied coating at elevated temperatures of about 120° C., and finally cooling it down to ambient room temperature of about 25° C. When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing solution co-application of the charge transport layer and ground strip layer and through drying/cooling process, upward curling of the multilayered photoreceptor is observed. This upward curling is a consequence of thermal contraction mismatch between the charge transport/ground strip layer in a typical electrophotographic imaging member device has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the charge transport layer/ground strip layer does therefore have a larger dimensional shrinkage than that of the substrate support as the imaging member web stock cools down to ambient room temperature, to result in tension stress/strain building-up and pull the imaging member upwardly. The exhibition of imaging member curling after completion of coating the charge transport layer/ground strip layer is due to the consequence of the heating/cooling processing step, according to the mechanism: (1) as the web stock carrying the wet applied charge transport layer/ground strip layer is dried at elevated temperature, dimensional contraction does occur when the wet charge transport layer/ground strip layer is losing the solvent due to evaporation at 120° C. elevated temperature drying, but at 120° C. the charge transport layer/ground strip layer are viscous flowing liquids after losing the solvent. Since its glass transition temperature (Tg) is at 85° C., the charge transport layer after losing of solvent will flow to re-adjust itself, release internal stress, and maintain its dimension stability; (2) as the charge transport layer now in the viscous liquid state cools down further and reaching its glass transition temperature (Tg) at 85° C., the charge transport layer instantaneously solidifies and adheres to the charge generating layer because it has then transformed itself from being a viscous liquid into a solid layer at its Tg; and (3) eventual cooling down the solid charge transport layer of the imaging member web from 85° C. down to 25° C. room ambient will then cause the charge transport layer to contract more than the substrate support since it has about 3.7 times greater thermal coefficient of dimensional contraction than that of the substrate support. This differential in dimensional contraction results in tension stress/strain built-up in the charge transport layer which therefore, at this instant, pulls the imaging member upward to exhibit curling. If unrestrained at this point, the imaging member web stock will spontaneously curl upwardly into a curl-up roll. By the similar fashion and same mechanism as described in the charge transport layer, the co-coated ground strip layer at the edge of the photoreceptor belt does also exhibit upward curling effect after solution co-coating with the charge transport layer and then through the drying/cooling processes. To offset the curling, an anticurl back coating is applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer, and render the imaging member web stock with desired flatness.
Curling of an electrophotographic imaging member web is undesirable because it hinders fabrication of the web into cut sheets and subsequent welding into a belt. To provide desirable flatness, an anticurl back coating, having an equal counter curling effect but in the opposite direction to the applied imaging layer(s), is therefore applied to the reverse side of substrate support of the active imaging member web to balance/control the curl caused by the mismatch of the thermal contraction coefficient between the substrate and the charge transport layer, resulting in greater charge transport layer dimensional shrinkage/contraction than that of the substrate after the heating/cooling processes of the charge transport layer coating. Although the application of an anticurl back coating is effective to counter and remove the curl, nonetheless the prepared flat imaging member web does have charge transport layer tension build-up creating an internal strain of about 0.27% in the charge transport layer and also in the ground strip layer as well. The magnitude of this charge transport layer internal strain build-up is very undesirable, because it is additive to the induced bending strain of an imaging member belt as the belt dynamically bends and flexes over each belt support roller during dynamic fatigue belt cyclic motion under a normal machine electrophotographic imaging function condition in the field. The summation of the internal strain and the cumulative fatigue bending strain sustained in the charge transport layer has been found to exacerbate the early onset of charge transport layer cracking, preventing the belt to reach its targeted functional imaging life. Moreover, imaging member belt employing an anticurl backing coating has added total belt thickness to thereby increase charge transport layer bending strain which then exacerbates the early onset of belt cycling fatigue charge transport layer cracking failure. The cracks formed in the charge transport layer as a result of dynamic belt fatiguing are found to manifest themselves into copy print-out defects, which thereby adversely affect the image quality printout on the receiving paper.
Various belt function deficiencies have also been observed in the common anticurl back coating formulations used in a typical conventional imaging member belt, such as the anticurl back coating does not always providing satisfying dynamic imaging member belt performance result under a normal machine functioning condition; for example, exhibition of anticurl back coating wear and its propensity to cause electrostatic charging-up are the frequently seen problems to prematurely cut short the service life of a belt and requires its frequent costly replacement in the field. Anticurl back coating wear under the normal imaging member belt machine operational conditions reduces the anticurl back coating thickness, causing the lost of its ability to fully counteract the curl as reflected in exhibition of gradual imaging member belt curling up in the field. Curling, caused by the internal strain in the charge transport layer and the ground strip layer, is undesirable during photoreceptor belt function, because different segments of the imaging surface of the photoconductive member are located at different distances from charging devices, causing non-uniform charging. In addition, developer applicators and the like, during the electrophotographic imaging process, may all adversely affect the quality of the ultimate developed images. For example, non-uniform charging distances can manifest as variations in high background deposits during development of electrostatic latent images near the edges of paper. Since the anticurl back coating is an outermost exposed backing layer and has high surface contact friction when it slides over the machine subsystems of belt support module, such as rollers, stationary belt guiding components, and backer bars, during dynamic belt cyclic function, these mechanical sliding interactions against the belt support module components not only exacerbate anticurl back coating wear, it does also cause the relatively rapid wearing away of the anticurl back coating produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance. Moreover, anticurl back coating abrasion/scratch damage does also produce unbalance forces generation between the charge transport layer and the anticurl back coating to cause micro belt ripples formation during electrophotographic imaging processes, resulting in streak line print defects in output copies to deleteriously impact image printout quality and shorten the imaging member belt functional life.
High contact friction of the anticurl back coating against machine subsystems is further seen to cause the development of electrostatic charge built-up problem. In other machines the electrostatic charge builds up due to contact friction between the anti-curl layer and the backer bars increases the friction and thus requires higher torque to pull the belts. In full color machines with 10 pitches this can be extremely high due to large number of backer bars used. At times, one has to use two drive rollers rather than one which are to be coordinated electronically precisely to keep any possibility of sagging. Static charge built-up in anticurl back coating increases belt drive torque, in some instances, has also been found to result in absolute belt stalling. In other cases, the electrostatic charge build up can be so high as to cause sparking.
Another problem, encountered in the conventional belt photoreceptors using a bisphenol A polycarbonate anticurl back coating that are extensively cycled in precision electrophotographic imaging machines utilizing belt supporting backer bars, is an audible squeaky sound generated due to high contact friction interaction between the anticurl back coating and the backer bars. Further, cumulative deposition of anticurl back coating wear debris onto the backer bars may give rise to undesirable defect print marks formed on copies because each debris deposit become a surface protrusion point on the backer bar and locally forces the imaging member belt upwardly to interferes with the toner image development process. On other occasions, the anticurl back coating wear debris accumulation on the backer bars does gradually increase the dynamic contact friction between these two interacting surfaces of anticurl back coating and backer bar, interfering with the duty cycle of the driving motor to a point where the motor eventually stalls and belt cycling prematurely ceases. Additionally, it is important to point out that electrophotographic imaging member belts prepared that required anticurl back coating to provide flatness have more than the above list of problems, they do indeed incur additional material and labor cost impact to imaging members' production process.
Thus, electrophotographic imaging member belts comprising a supporting substrate, having a conductive surface on one side, coated over with at least one photoconductive layer (such as the outermost charge transport layer) having a co-coated adjacent ground strip layer at one edge of the belt, and the application on the other side of the supporting substrate with a conventional anticurl back coating that does exhibit deficiencies which are undesirable in advanced automatic, cyclic electrophotographic imaging copiers, duplicators, and printers. While the above mentioned electrophotographic imaging member belts may be suitable or limited for their intended purposes, further improvement on these imaging member belts are needed. For example, there continues to be the need for improvements in such systems, particularly for an imaging member belt that has sufficiently flatness, superb wear resistance, nil or no wear debris, ease of belt drive, and eliminates electrostatic charge build-up problem, even in larger printing apparatuses. With many of above mentioned shortcomings and problems associated with electrophotographic imaging member belts having an anticurl back coating now understood, therefore there is a need to resolve these issues through the development of a methodology for fabricating imaging member belts that produce improve function and meet future machine imaging member belt life extension need. In the present disclosure, a charge transport layer material reformulation and a ground strip layer re-composition method and process of making a flexible photoreceptor belt free of the mentioned deficiencies have been identified and successfully demonstrated through the preparation of anticurl back coating-free photoreceptor belt. The improved curl-free photoreceptor belt, having absolute flatness in belt and cross-belt directions without the need of a conventional anticurl back coating, suppresses abrasion/wear failure, extends the charge transport layer cracking, and provides excellent dynamic belt motion quality under a normal machine functioning condition in the field will be described in detail in the following.
Relevant disclosures of flexible electrophotographic imagine member designs and their preparation method are listed below.
Conventional photoreceptors are disclosed in the following patents, a number of which describe the presence of light scattering particles in the undercoat layers: Yu, U.S. Pat. No. 5,660,961; Yu, U.S. Pat. No. 5,215,839; and Katayama et al., U.S. Pat. No. 5,958,638. The term “photoreceptor” or “photoconductor” is generally used interchangeably with the terms “imaging member.”
Horgan et al., U.S. Pat. No. 4,664,995 issued on May 12, 1987, discloses an electrostatographic imaging member comprising at least one imaging layer capable of retaining an electrostatic latent image, a supporting substrate layer having an electrical conductive surface, and an electrically conductive ground strip layer adjacent the electrostographic imaging layer and in electrical contact with the electrical conductive layer, the electrical conductive ground strip layer comprising a film forming binder, conductive particles and crystalline particles dispersion in the film forming binder and a reaction product of a bi-functional chemical coupling agent with both the film forming binder and the crystalline particles. The imaging member may be employed in an electrostatographic imaging process.
Yu, U.S. Pat. No. 6,183,921 issued on Feb. 6, 2001, discloses a crack resistant and curl-free electrophotographic imaging member design which includes a charge transport layer comprising an active charge transporting polymeric tetraaryl-substituted biphenyldiamine, and a plasticizer.
Yu, U.S. Pat. No. 6,660,441, issued on Dec. 9, 2003, discloses an electrophotographic imaging member having a substrate support material which eliminates the need of an anticurl backing layer, a substrate support layer and a charge transport layer having a thermal contraction coefficient difference in the range of from about −2×10−5/° C. to about +2×10−5/° C., a substrate support material having a glass transition temperature (Tg) of at least 100° C., wherein the substrate support material is not susceptible to the attack from the charge transport layer coating solution solvent and wherein the substrate support material is represented by two specifically selected polyimides.
In U.S. Pat. No. 7,413,835 issued on Aug. 19, 2008, there is disclosed an electrophotographic imaging member having a thermoplastic charge transport layer, a polycarbonate polymer binder, a particulate dispersion, and a high boiler compatible liquid. The disclosed charge transport layer exhibits enhanced wear resistance, excellent photoelectrical properties, and good print quality.
Although the above-described electrophotographic imaging member belts comprise a flexible supporting substrate, a conductive surface on one side, coated over at least one photoconductive layer (such as the outermost charge transport layer) with a co-coated adjacent ground strip layer at one edge of the belt, and coated on the other side of the supporting substrate with an anticurl back coating have offer some degree of improvements, they still exhibit deficiencies which are undesirable in advanced automatic, cyclic electrophotographic imaging copiers, duplicators, and printers.
While the above mentioned electrophotographic imaging member belts may be suitable or limited for their intended purposes, further improvement on these imaging member belts are definitively required. For example, there continues to be the need for improvements in such systems, particularly for developing an imaging member belt that is absolutely curl-free and minimizes wear debris build-up, and enhances toner image transfer efficiency to receiving papers with improved image copy print-out quality in printing apparatuses and xerographic machines. In the present disclosure, a charge transport layer reformulation and a ground strip layer re-composition were successfully demonstrated by plasticizing both these layers through the incorporation of a specifically selected plasticizing liquid to eliminate the charge transport layer and the ground strip layer internal stress/strain build-up so that the resulting imaging member belt thus obtained exhibits nil or no upward curling without the application of an anticurl back coating. That means the prepared curl-free flexible imaging member belt of this disclosure has total flatness in either the belt or the trans-belt direction to impact dynamic belt motion quality improvement. Additional, without the need of an anticurl back coating, the disclosed imaging member belt thus prepared is also an effective production cost cutting measure.